This invention relates to the field of digital data transmission and, more particularly, to the self-synchronous scrambling of a packet based transmissions using non-return-to-zero (NRZ) line coding that permits the recovery of the reference clock at the receiver.
NRZ encoded transmission systems are typically designed so the timing reference clock at the receiver is recovered solely from transitions in the received data signal. Information is reliably communicated over such systems if the transition density of the transmitted data sequence is sufficiently high. If the data sequence transition density is not adequate, the recovered timing reference clock in the receiver will not track the transmitter timing reference accurately enough to receive the data sequence error free. When the transition density is too low, the receiver timing reference becomes “unlocked” with respect to the transmitter timing reference, and communication between the transmitter and receiver is lost.
In order to ensure that adequate data transition density exists, many NRZ encoded transmission systems rely on scrambling to randomize the data prior to transmission. The SONET format, as described in GR-253-CORE “Synchronous Optical Network Transport Systems: Common Generic Criteria”, Revision 1, December 1997, Bellcore, is an example of such a system. The frame synchronous SONET scrambler described in the above publication works well for Time Division Multiplexed (TDM) based payload mappings that interleave data from multiple sources into a single SONET frame. For payload mappings that accept data from a single source, the SONET frame synchronous scrambler may not be adequate. The SONET format suffers from two liabilities: 1) it is reset to the same value at the beginning of each SONET frame; and, more importantly, 2) the length of the sequence before repeating is only 127 bits.
Publication RFC-1619, “PPP over SONET/SDH”, Issue 1, May 1994, Internet Engineering Task Force, defines a direct mapping of the HDLC (high-level Data link control) encapsulated packet based point-to-point protocol (PPP) into the SONET payload. As is noted in “Self-Synchronous Packet Scrambler”, U.S. Pat. No. 5,835,602, invented by S. Lang, a malicious user may generate packets consisting of the SONET frame synchronous scrambler sequence. If such packets are transported using the mapping defined in RFC-1619, there is a non-trivial probability that the packet sequence would be aligned with the frame-synchronous SONET scrambler resulting in long sequences of ones or zeros that could disrupt the receive clock recovery circuit.
U.S. Pat. No. 5,835,602 describes a method of reducing the probability of the above-described disruption, which involves adding a self-synchronizing scrambler after the HDLC (protocol for X.25 packet switching networks) packet generation, but before the SONET frame generator. This provides protection as long as the malicious user has no knowledge of the state of the self-synchronizing scrambler. However, a user may have knowledge of the scrambler state at the start of transmission. The self-synchronizing scrambler is usually initialized to a pre-defined state (such as all ones). Then, if only HDLC idle flags are passed through the scrambler until the packet transmission is initiated, the self-synchronizing scrambler will be in one of a small number of states when packet transmission begins. Thus, a malicious user could still disrupt transmission with a non-trivial probability of success.
It would be advantageous if an improved method of SONET data scrambling could be derived that reduced the probability of a user guessing the pseudo-random scrambling pattern. It would be advantageous if the data could be scrambled in a way that would prevent a malicious user from transmitting a long string of “0”s or “1”s which prevent clock recovery.
It would be advantageous if the overhead sections of packetized data could be scrambled to reduce the possibility of guessing the scrambling code during initialization periods, or periods where no information is sent. It would be advantageous if the improved method of data scrambling could be made backward compatible with contemporary encryption schemes.
Accordingly, an improved scrambling system for the NRZ format communications is provided. The system is applicable to the transmission of HDLC data packets according to SONET protocols. The system comprises a frame generator to accept information to be transmitted. The frame generator organizes the information into frames, including both the information and system overhead. The input information may already be organized in another, distinct, communication format.
A self-synchronous scrambling circuit scrambles the frame input. That is, scrambling occurs after the information is organized into frames with the frame overhead. After transmission, a self-synchronous de-scrambling circuit recovers the received scrambled frames to provide received frames of information and overhead. A frame terminal removes the overhead information associated which each frame to provide the transmitted, or recovered information.
The frame generator divides each frame into time multiplexed sections. The information and overhead are loaded into different sections of the frame. The frame generator also provides the timing data, corresponding to the information and overhead frame periods, to the scrambler. The scrambler has the capability, then, of optionally scrambling frame sections in response to the timing data. Typically, the information portion of the frame is always scrambled. Optionally, the system has the potential of scrambling the overhead, except perhaps for the parts of the overhead that are required to locate frame boundaries. Further, the sub-sections of the overhead that are scrambled can be varied in patterns, providing yet another layer of system scrambling.
Likewise, the frame terminal has an output connected to the de-scrambler to provide timing data corresponding the frame information and overhead sections. The de-scrambler can be programmably engaged to de-scramble sections of the overhead in response to higher layer of scrambling protocol.
An improved method of scrambling communications in a NRZ formatted coding system is also provided. The method includes the steps of:                a) accepting information to be transmitted;        b) organizing the information into frames including information and overhead sections;        c) scrambling the frames;        d) de-scrambling the frames; and        e) recovering the information from the frames (removing the overhead).        
Step b) includes generating timing data to signal the occurrence of the information and overhead sections of the frames, and Step c) includes scrambling the frames in response the timing data signals of Step b). Likewise, Step e) includes generating timing data to signal the occurrence of the information and overhead sections of the received frames, and Step d) includes de-scrambling the received frames in response the timing data signals of Step e).
In accordance with a higher layer of transmission encryption, Steps c) and d) can selectively scramble and de-scramble partial sections of overhead. Further, the overhead sub-section being scrambled may be constantly varied in a pattern that is decipherable to the message receiver, but cannot be known, before transmission, by a communication sender.